Primary electrochemical cells that use the metals aluminum, lithium, magnesium, or zinc as a negative electrode, and rechargeable electrochemical cells that use an aluminum, cadmium, lead, lithium, magnesium, zinc, or metal hydride forming hydrogen storage alloy negative electrode are well known in the art. In fact these materials represent a large percentage of battery materials used in consumer and automotive batteries.
When electrochemical storage batteries which employ these materials are subjected to analysis to determine the cause of failure, it has been found that the failure mode is usually the result of degradation of the negative electrodes. This degradation has often been ascribed to dendrite formation which promotes internal shorting, and to the growth of an oxide or hydroxide film on the surface of the negative electrode. These surface films reduce the active area for the reduction/oxidation reaction to occur. Since the total current has to be distributed over a smaller total area, the current density on the active surface increases. As a consequence, the rate of formation of the irreversible film layer increases. The internal resistance of the electrode also increases, further hastening failure of the electrode.
Prior attempts to address these problems have focused mainly on the addition, through high temperature alloying, of more and more modifier elements to the electrochemnical storage material which makes up the negative electrode. For example, many current examples of metal hydride materials include ten or more components mixed in varying ratios. As with any alloy, adding new elements increases the complexity of the formation process, and adds to the cost of the overall material.
Accordingly, there exists a need to provide a means to reduce the formation of dendrites and of oxides and hydroxide films on the surface of negative electrodes of electrochemical cells. The means for reducing film formation should be relatively simple, and not necessitate the use of additional elements by high temperature alloying.
Methods for protecting the negative electrode in an electrochemical cell without alloying have been described in U.S. Pat. No. 4,315,829 by Duddy et al. in which lead electrodes are coated with a polymeric material in order to inhibit shedding, in U.S. Pat. No. 5,185,221 by Rampel in which hydride materials are coated with a relatively thick layer of carbon to inhibit the growth of oxide films, and U.S. Pat. No. 5,451,474 by Han Wu et al. where hydride materials are coated with a passivating layer of platinum or palladium to facilitate hydrogen adsorbtion and inhibit the growth of oxide an hydroxide films. Fauteux et al. in U.S. Pat. No. 5,434,021 describes a method by which dendrite formation on a lithium metal anode may be inhibited by coating with a polymer. However, none of the processes described, provide the fine degree of control, obtained in the instant invention, over the thickness of the protective passivating material which is crucial to the efficient flow of ions to the primary material, nor do they allow for the beneficial, composite characteristics made possible by the addition of inclusion materials within layers of the protective passivating material. Further, none of the methods disclosed describe processes by which a wide variety of common negative electrode materials may be induced to exhibit composite characteristics, such as both excellent electrochemical storage capacity and high resistance to corrosion, without high temperature alloying.
Hydrogen occluding materials such as metal hydrides have been suggested as suitable materials for the storage of hydrogen gas for future transportation applications.
One of the primary purposes of alloying materials with hydride forming metals to form gaseous hydrogen storage materials is to enhance interstitial spacing between the hydrogen occluding particles, thus enhancing the passage of hydrogen gas to the adsorption sites. Another of the primary purposes of alloying materials with hydride forming metals is to provide sites within the alloy which enhance its catalytic ability. The alloying techniques utilized to achieve these goals are complex and expensive. Accordingly a need exists to achieve these goals by less complex and expensive means.
In addition, hydrogen occluding materials may have significant potential in applications where the separation and recovery of gaseous hydrogen or hydrocarbons from mixed gaseous environments is desirable, such as in the recovery of waste hydrogen or unburned hydrocarbons in combustion exhaust streams or the like. However, metal hydrides used for storage of gaseous hydrogen are often highly reactive with atmospheric gasses such as nitrogen, oxygen, and carbon dioxide and must be reconditioned after exposure to these gasses, thus making the handling of these materials both expensive and difficult and there use in the separation and recovery of hydrogen from a mixed gas environment, impractical. Therefore, a need exists so that hydrogen occluding materials may be exposed to atmosphere and mixed gas environments, and utilized without reconditioning.
Over the past several years, metal hydride cells have gained widespread market acceptance due to the fact that they incorporate highly desirable performance characteristics. Examples of these desirable characteristics include high charge acceptance, relatively long-cycle life and operation over a wide range of temperatures. Each of these performance characteristics represent improvements over the nickel cadmium and other battery systems known in the prior art.
Typically, the metal hydride hydrogen storage alloy electrode is the negative electrode in a hydrogen storage system. The negative electrode material (M) is charged by the electrochemical absorption of hydrogen, and the electrochemical evolution of a hydroxyl ion. The reaction which takes place at the metal hydride electrode may be described according to the following formula; ##STR1##
The reaction that takes place at the positive electrode of a nickel metal hydride cell is also a reversible reaction. In the case of a nickel oxy-hydroxide electrode, the positive electrode reaction is as follows; EQU Ni(OH).sub.2 +OH.sup.- &lt;- - - &gt;NiOOH+H.sub.2 O+e.sup.-
The negative electrode of most metal hydride electrochemical cells can be characterized by one of two chemical formulas: The first is AB.sub.2, which describes TiNi type battery systems such as described in, for example, U.S. Pat. No. 5,277,999. The second formula is AB.sub.5 which describes LaNi.sub.5 type systems as described in, for example, U.S. Pat. No. 4,487,817.
However, the power density of metal hydride cells is not as great as in some other types of cells, notably nickel cadmium. Accordingly, metal hydride cells have not been appropriate for several applications, such as power tools. Therefore, a needs exists for metal hydride cells having relatively high power densities and capacities.
The inventor has discovered that all of these goals may be achieved by the composite materials revealed.
A crucial element for the manufacture of the composite relies on the ability of one component of the composite to form defined layers of a specific molecular thickness, within which various materials may be included, while homogeneously dispersed in a suspension, prior to compounding with another component of the composite which acts as the primary electrochemical or hydrogen storage material.
Dines et al. in U.S. Pat. Nos. 4,299,892, and 4,323,480, and Morrison et al. in U.S. Pat. Nos. 4,853, 359 describe methods whereby a layer or layers of a transition metal di- and poly-chalcogenides, formed in a homogenous layered suspension may be applied to a support substance selected from the oxides or hydrous oxides of various elements. Further, Dines describes the beneficial characteristics which may be achieved when these materials are utilized as cathodes, that is, as the positive electrode, in lithium cells. Miremadi and Morrison in J. Appl. Phys.67(3), Feb. 1, 1990 describe how alternating layers of dissimilar layered dichalcogenides with and without an inclusion material may be stacked and supported by an oxide substrate. The rational stated, behind using oxides or hydrous oxides as support structures, was that the negatively charged layered dichalcogenides would adhere to the positively charged oxide or hydrous oxide.
This method of forming a bond between opposite charged materials, limits the range of materials which may be compounded and it would be beneficial if alternate methods which allow a more diverse group of materials to be compounded could be developed. Further, none of these methods teaches the benefits to performance that are achieved by compounding the materials, within these layered suspensions, with the wide variety of electrochemical anode and hydrogen occluding materials described in the present invention.
Divigalpitya et al. in U.S. Pat. Nos. 4,996,108 and 5,279,720 describes methods by which a metal dichalcogenide may be applied as a single layer film to various substrates and methods for electrodepositng these materials. Winter et al. in U.S. Pat. No. 5,425,966 discloses methods for producing thin film coatings of selected dichalcogenides through chemical vapor deposition.
However, they do not describe the cost effective methods revealed in the instant invention for compounding the various components of the composites disclosed, by mixing in a manner by which a precise thickness of the layered materials may be applied, nor do they disclose the methods described in the instant invention for enhancing the adhesion of materials within the composite to each other through the application of various heat treatments. Further, none of the methods previously described, reveal processes by which successive layers of these materials with different inclusions contained within the layers may be applied to a substrate, nor do they disclose methods by which materials that are reactive with water, such as metal hydrides, may be compounded with layered materials which have been homogeneously dispersed in water.